Aphids

In general, aphids (order Homoptera) are small, gregarious insects with soft, pear-shaped, green, yellow, black or colorless bodies. Their long, slender legs allow for slow movement. Most species possess "cornicles" a pair of tube-like, truncate, or porelike structures on the dorsal, posterior section of the body. Adults may be wingless or have four transparent, delicate wings. The most commonly occurring aphids locally are several species of giant conifer aphids of the genus Cinara, and the Cooley spruce gall aphid Adelges cooleyi. Other aphids that have been pests are the green spruce aphid, Elatobium abietinum; the primitive woolly aphid, Mindarus obliquus; and two conifer root aphids, Pachypappa tremulae and Prociphilus xyloster. The balsam woolly aphid, Adelges piceae (also known as the balsam woolly adelgid), although a concern because of quarantine regulations, has not been reported at any local nurseries.

Hosts and damage

Each tree species grown in local nurseries is a host for at least one species of giant conifer aphid. Great numbers of these large (3-5 mm long), dark-colored, long-legged aphids (Figure 89) feed gregariously on seedling stems, where they may cause stunting and chlorosis. Their 1 mm long, shiny black, oval-shaped eggs are usually laid individually on needles at the end of summer. Growers should watch for eggs when lifting seedlings, because they will accompany seedlings to planting sites. Adults characteristically produce copious amounts of honeydew on which sooty moulds grow. As well, honeydew attracts ants and wasps, which are good indicators of aphid infestation.

The coniferous root aphid, Pachypappa tremulae, previously called Rhizomaria piceae, has been a pest at several British Columbia nurseries, particularly near Prince George. Infestations (Figure 90) are most common on container seedlings. Bareroot stock is rarely affected. This aphid's major hosts are container-grown spruce, potted spruce grafting stock, and sometimes pine, larch, and Douglas-fir. To date, infestations on otherwise healthy seedlings have caused no damage; this may not be true for stressed seedlings. Little is known about this aphid's life history, damage, or distribution. The aphids, which are usually most abundant on the exterior roots in the upper portion of the container cavity may go unnoticed until the seedlings are lifted. They secrete white waxy strands that might be mistaken for mycorrhizae. Part of the aphid's life cycle occurs on trembling aspen Populus tremuloides leaves, which may explain the predominance of infestations at nurseries in the north central Interior.

The woolly stage of the Cooley spruce gall aphid can be serious on Douglas-fir and spruce (white, Engelmann, and Sitka) seedlings. The aphids, approximately 1 mm long, are covered in tufts of a white cottony woollike substance and can be found on the lower surface of needles (Figure 91) and shoots year-round. Their feeding (Figure 92) causes mottled, twisted needles, and severe infestations cause stunting and defoliation. Another stage of the aphid can occasionally cause galls on transplant or grafted spruce stock that is at least 3 years old and thus large enough to sustain the galls.

The primitive woolly aphid attacks only spruce seedlings and has been a problem at several British Columbia nurseries. The aphids occur in small colonies at the seedling tip, appearing as white woolly masses. These aphids can cause terminal mortality and deformation. There is little information on this aphid's life cycle or host range.

The balsam woolly aphid has become established in British Columbia and can seriously damage all true firs (Figure 93). It occurs throughout the Lower Mainland and in the southern half of Vancouver Island. To date, it has only been reported in forest stands, never in forest nurseries. Such an infestation is a potential threat to forested areas if infested seedlings are outplanted to reforestation sites. Therefore, all nurseries in the province growing Abies must obtain a permit annually from the Ministry of Agriculture and Fisheries to satisfy quarantine regulations.

The green spruce aphid (Figure 94) is a serious pest of spruce species along coastal British Columbia - particularly of Sitka spruce - and may occur on pine and Douglas-fir. Adults are green, 1 mm long with long cornicles. Preferring older needles, they are usually found on the lower shaded needles rather than on leader or growing tips. Initial damage results in mottled needles, followed by chlorosis and needle drop. Severe infestations can lead to complete defoliation and seedling death. They overwinter as adults on foliage and, under mild winter conditions, continue to reproduce and feed. Nursery workers must therefore, be aware of green spruce aphid populations throughout the year.

Life history*

The life history of aphids, including the ones discussed here, is complex. In general, most aphids pass through several generations per year. Populations fluctuate widely and quickly. Some juveniles hatch from eggs, others may be born alive. Overall, except for size, adults and juveniles are similar in appearance. Parthenogenesis is common; however, the last seasonal generation is usually sexual. Overwintering is usually as eggs. Some aphids complete their life cycle on two kinds of plants. Even this relationship may be optional. For example, the Cooley spruce gall aphid normally requires Douglas-fir and spruce to complete its life cycle, but certain wingless generations may live only on spruce or Douglas-fir.

*Because of the variation in the life history of the aphids described here, a diagram is not included.

Management

Aphid damage usually does not occur until populations are large; however, population build-up can be fast and erratic. Predicting aphid outbreaks is difficult in nurseries, but the presence of wasps or ants is a good indicator of aphids. Wasps feed on both honeydew and aphids, and can effectively control small populations. However, it is virtually impossible to exclude aphids from the nursery, as they are frequently blown in by the wind. All these factors justify waiting until populations have been detected before controls such as insecticidal soaps or insecticides are applied. The probability and intensity of A. cooleyi outbreaks may be decreased by excluding spruce in windbreaks or the nursery periphery adjacent to Douglas-fir seedbeds.

Selected References
 

Bradley, G.A. 1961. A study of the systematics and biology of aphids of the genus Cinara Curtis in Canada. Ph.D. thesis. McGill Univ., Montreal, Que.

Harris, J.W.E. 1978. Balsam woolly aphid. Pac. For. Res. Cent., For. Pest Leafl. 1, Environ. Can., Can. For. Serv., Victoria, B.C.

Koot, H.P. 1983. Spruce aphid in British Columbia. Pac. For. Res. Cent., For. Pest Leafl. 16. Environ. Can., Can. For. Serv., Victoria, B.C.

Wood, C. 1977. Cooley spruce gall aphid. Pac. For. Res. Cent., For. Pest, Leafl. 6, Environ. Can., Can. For. Serv., Victoria, B.C.

Summary

Aphids